I highly recommend learning TeX and LaTeX. TeX is the typesetting system created by Donald Knuth. TeX can be converted to HTML, PDF, and countless other formats with ease. Personally, I use LaTeX, which is a bunch of macros and extensions to TeX that let you avoid a lot of work by combining a group of commands commonly used together into one.

I would suggest Markdown. It may be a bit geeky (I'm a programmer/designer.), but it's what I write everything in now. It's a minimal syntax markup that you write in plain text, and then can convert it to HTML, and others. It's often used by bloggers, since a lot of the blogging software out there will let you write in Markdown, and then the blog system will convert it to the correct HTML to display on one's blog.

I'd like to suggest Google Docs. It's a simple, usable word processor which allows you to "Download As" several formats (ODT, PDF, RTF, Text, Word, and HTML (zipped)). The only oddity is in the content licensing.

Essentially, you're granting Google rights to redistribute your work free of royalties "for the sole purpose of enabling Google to provide you with the Service in accordance with the Google Docs Privacy Policy.".